Beam-forming refers to a process of computing an optimal weighting vector according to parameters and specifically refers to the process of realizing an optimal (sub-optimal) combination or an optimal (sub-optimal) allocation of signals after measuring and estimating parameters. The beam-forming technology uses an antenna array to aggregate signal power to a narrow beam so as to improve transmission efficiency of the antenna, reliability of the wireless link and repeat usage ratio of the frequency. Beam-forming algorithm can be designed based on different principles and generally there are two kinds of principles: maximum user power principle and maximum user CIR (Carrier to Interference ratio) principle. The beam-forming algorithm based on the maximum user power principle makes the receive power of the user maximum so that the receive SNR is improved. The beam-forming algorithm based on the maximum user CIR principle makes the receive CIR of the user maximum so that a null is formed in the direction of an interference signal and the directional interference is suppressed. Therefore, in order to suppress the interference signal, the beam-forming algorithm based on the maximum user CIR principle is often adopted.
In the conventional technology, the main idea of the beam-forming algorithm based on the maximum user CIR principle is that obtaining channel impulse responses of an expected user signal and interference signal respectively according to the channel estimation for the expected user signal and the interference signal in the received signals; obtaining array correlation matrix RS(k) of the expected user signal and the array correlation matrix RI(k) of the interference signal; and computing the beam-forming weighting coefficient of the array antenna according to the obtained array correlation matrixes.
Generally, the CIR received by user k can be expressed in formula (1).
                              γ                      (            k            )                          =                                            W                                                (                  k                  )                                ⁢                H                                      ⁢                                          R                S                                  (                  k                  )                                            _                        ⁢                          W                              (                k                )                                                                        W                                                (                  k                  )                                ⁢                H                                      ⁢                                          R                I                                  (                  k                  )                                            _                        ⁢                          W                              (                k                )                                                                        (        1        )            
w(k) is the beam-forming weighting coefficient vector, RS(k)=h(k)*h(k)T is the array correlation matrix of the expected user signal, h(k) is the array channel impulse response vector of user
  k  ,                    R        I                  (          k          )                    _        =                  ∑                  ∀                      m            ∈            I                              ⁢                        h          I                                    (              m              )                        *                          ⁢                  h          I                                    (              m              )                        ⁢            T                              is the array correlation matrix of the interference signal and I is a set of interference signals.
For the beam-forming algorithm based on the maximum user CIR principle, since the optimization principle of such kind of beam-forming algorithm leads to maximum CIR and has no limits on the receive power of the user, it may cause that the main lobe of the formed beam may not direct to the direction of the expected user signal. The reason is that according to the formula (1), the interference suppression algorithm is realized by improving the power of the expected user signal and reducing the power of the interference signal. However, for an array antenna with N antenna units, the power of the expected user signal can be improved at most by N times; however, for the interference suppression, the degrees of freedom of the array antenna is N−1, so if the number of the interference signals is not larger than N−1, the interference signal can be set to zero theoretically. In addition, the expected user signal and the interference signal cannot be completely orthogonal. Thus, there is a confliction between making the expected user signal maximum and setting the interference signal to zero and the direction of the formed beam based on the maximum CIR will be deviated.
Therefore, because of the limit of the degrees of freedom and the orthogonality, the beam-forming algorithm based on the maximum CIR principle cannot guarantee the direction of the main lobe of the beam and the power of the expected user signal, which not only reduces the power but also increases the susceptibility to the error of the interference signal estimation.